This invention relates to foundry sand compositions for preparing mold and cores. More particularly, this invention relates to foundry sand compositions which have better workability characteristics and produce fewer defective moldings than foundry sand compositions of the prior art. Still more particularly, this invention relates to the use of water-soluble graft copolymerization products of acrylic acid and a minor amount of a water-soluble polyhydroxy polymeric compound, and water-soluble salts thereof, to impart superior workability characteristics to foundry sand compositions.
Conventional foundry molding sands are prepared by adding a bonding clay, such as a bentonite or fire clay or a combination of such clays, and a critically controlled amount of water to a classified sand which may be of a washed, crude or reclaimed variety. It is also common practice to employ an organic binder such as cereal or dextrine, and a combustible material such as sea coal. Deficiencies in the molding and working properties of the foregoing general type of molding compositions have presented a constant challenge to the foundry industry.
The foundry industry, therefore, has attempted numerous variations in foundry sand compositions with respect to the binder employed, but it has been a general result that the different binders utilized have improved certain molding characteristics while degrading others. The different binders utilized in foundry sand compositions have included synthetic water-soluble polymers. The polymers of the prior art which have been used as binders in foundry sand mold and core compositions have been described as improving properties of the compositions, e.g., as giving good bakeability, good permeability to gases generated during the baking operation, good blendability or the ability to mix well and deposit well upon blowing, and the like. Many of the polymers of the prior art having the above-mentioned properties are deficient in one or more of other essential properties, namely, good green strength, good hot strength, and good deformation characteristics. It was, therefore, impossible to select a single polymer which would produce a good balance of the essential properties of a foundry sand composition.
As illustrative of the types of polymers suggested for use in foundry sand compositions, U.S. Pat. No. 2,817,128 teaches the use of a water-soluble polyelectrolyte, as for example, a polyacrylic acid, instead of organic binders such as cereal and dextrine. These polyelectrolytes are described as improving the workability and flowability of the foundry sand in addition to making the water content of such compositions a less critical variable. An improvement over the use of these polyelectrolytes is described in U.S. Pat. No. 3,104,230 which teaches the use of certain inorganic compounds in combination with the polyelectrolytes to impart superior hot strength properties to the sand compositions. These inorganic compounds include phosphoric acid and the various alkali metal and alkaline earth metal salts of the oxy-acids of phosphorus and the oxy-acids of sulfur, such as tetrasodium pyrophosphate.
Although the aforementioned inorganic compounds provide improvements over the sole use of the polyelectrolytes as binders, the foundry sand compositions of U.S. Pat. No. 3,104,230 still suffer from certain deficiencies. For example, while the use of the aforementioned inorganic compounds improved the hot strength of the foundry sand compositions, the hot strength is not improved to the extent desirable. The utilization of the invention of U.S. Pat. No. 3,104,230 suffers from the further disadvantage that the patent teaches it is necessary that a major proportion of the total clay in the composition be of the Western bentonite type. This limitation presents an economic disadvantage in areas where the sub-bentonitic type of clay is more readily available.
As illustrative of another type of binder suggested for use in foundry sand compositions, U.S. Pat. No. 3,179,523 teaches the use of a magnesium or zinc oxysalt cement in combination with an organic binder such as a water-soluble polysaccharide or water-soluble polysaccharide derivative.